This invention relates to suspension struts for use in self-levelling suspension systems of vehicles.
A typical self-levelling suspension system comprises an accumulator for storing fluid under pressure, a pump for maintaining the reservoir full of fluid under pressure, and, for each wheel of the vehicle, a suspension strut which transfers a vertical load between the body of the vehicle and the wheel, and a levelling valve which controls the admission of fluid under pressure from the reservoir to, or the release of fluid from the cylinder of a piston and cylinder assembly forming part of the strut, the levelling valve being arranged to keep the height of the vehicle body constant despite changes in load. Such systems often use oil as the fluid, and in this case each suspension strut usually includes means arranged to damp movements of the strut by causing the oil to flow through a constriction on extension or contraction of the strut. Also, the working cylinder of the strut may contain, or communicate with a vessel containing, a quantity of air or other gas, to provide the resilience needed in the suspension system. This makes it possible to dispense with the steel springs usually needed in a suspension system.
However, although suspension systems of the type described have a generally satisfactory performance, they suffer from certain disadvantages which result from the design of the struts used. One disadvantage is that, because the fluid in the interior of the working cylinder is supplied from the rest of the suspension system, contaminants may be introduced into the working cylinder with this oil. It is also possible for contaminants to be introduced into the working cylinder during fitting of the strut to the vehicle.
Another disadvantage is that the manufacturer of the strut cannot conveniently supply the strut filled with oil. Thus, although it is possible for the manufacturer to test the strut before delivery, it is not possible to leave the oil filling undisturbed between testing and fitting to a vehicle, and therefore testing does not guarantee reliable operation. Also, testing of the previously-known struts requires them to be connected to a fluid pressure system similar to the suspension system in which they are to be used.
Equally, it is not convenient for the manufacturer of the strut to supply the strut with the space which, in operation, forms a gaseous suspension spring, already pressurized, since it would be almost impossible to fit the strut to the vehicle and connect it to the fluid pressure system without upsetting the pressurization of this space.